This patent pertains to the reduction of elastomeric particles to a very small size by abrading or grinding the particles between rotating milling wheels.
It has been found particularly advantageous in recycling and reusing elastomeric products to reduce the size of this material to very small size particles; such particles have been discovered to be chemically more reactive and mechanically easier to dissolve into various mixes or recycled uses. Such very small particles are here defined as being particles that will pass through a minus 50 mesh or finer. Since the particles are of very irregular outline, this is the preferred method of stating sizes; they have no defined orientation, and thus no external dimensions can be specified. A wide range of rubber or synthetic rubber products may be so treated and reclaimed. Such materials include natural or synthetic rubber scrap, automotive tire scrap, and various polymers and plastics. The most common such materials are natural or synthetic rubbers.
Various methods have been suggested in the prior art for reducing fine sized elastomeric particles as part of a reclaiming or recycling process including cryogenic cracking of the particles and various grinding or chopping methods.
However, the best method now known for producing, in the greatest quantity and at less cost, fine elastomeric particles has been by milling the rubber between horizontal grinding stones in a horizontal grinding mill. This technique has been well developed in the flour, paper pulp industry and the paint pigment compounding industry.
Such methods for horizontal mill grinding of rubber are shown in, for example, U.S. Pat. No. 4,535,941 to Brubaker, et al which shows a method of horizontal grinding rubber pellets. A mixed rubber fluid slurry is formed of coarse rubber pellets in a carrier liquid, typically water. This slurry is pumped under pressure into a zone between two milling wheels, and the rubber particles sizes are reduced in the region between the faces of these milling wheels. The milling wheels are in the form of two disc shaped grinding stones, facing and opposed to each other; one stone is fixed and the other stone rotates with respect to the fixed stone. The two milling wheels are pressed upon the pellets and slurry with considerable force in an effort to reduce the pellets to a fine state in a single pass.
U.S. Pat. No. 4,625,922 to Brubaker shows another method for grinding rubber pellets, utilizing elevated temperatures and pressures in an attempt to produce a fine rubber grind.
It has been found in practice that the above processes do not produce a uniformly fine grind and in fact, seldom produce particles in the claimed 50 micron range.
Further, adjustment of the grinding mills, which are the same mills widely used in the paint pigment compounding industry, is extremely critical under the methods previously disclosed. For instance, the first Brubaker patent requires pressure of over 2000 pounds be imposed between the milling wheels; the slightest interruption of slurry flow under these conditions would result in the instant contact of the milling wheels and their seizure and destruction. The friction and energy introduced to the slurry by this milling process raises the slurry to relatively high temperatures, and much of the carrier liquid is lost as steam, especially if the pressure on the slurry is released. If this steam flash over occurs between the grinding stones the slurry becomes a largely dry rubber mass, also inhibiting grinding.
The attempt to simultaneously control motor current and stone pressure as variables results in wide swings in through-put and an extreme variability in the quality of the resulting ground product. It has proven in practice quite difficult to obtain sustained high uniform quality production rates using the processes heretofore described.